Exposure control is, inter alia, required in the context of digital photography, where a target image is captured through a lens by using an image sensor, such as for instance a Charge Coupled Device (CCD) or Complementary Metal-Oxide Semiconductor (CMOS) image sensor. Such image sensors are for instance installed in digital cameras such as Digital Still Cameras (DSC) or in electronic devices that offer a camera function, such as for instance a mobile phone. Therein, exposure control determines the exposure time during which the target image is captured by the image sensor.
Exposure control is particularly demanding if the dynamic range of an image sensor is limited. Therein, the dynamic range of an image sensor may be understood as the ratio between high and low extremes in a set of intensity values that can be captured by the image sensor. In case of limited dynamic range, the problem arises that details in the bright area have to be traded against details in the dark area. With a large exposure time, the pixels of the image sensor associated with bright parts of the captured image will be driven into saturation, so that there are no details in the bright part, whereas with a small exposure time, the pixels of the image sensor associated with the dark parts of the captured image will only receive small or none activation at all, so that there are no details in the dark part.
To avoid image degradation due to limited dynamic range, High Dynamic Range (HDR) cameras have been proposed, providing dynamic ranges of 12-20 bits per color component compared to the 6-12 bits per color component offered by normal cameras.
A first type of HDR cameras achieves increased dynamic range by providing an image sensor with high and low sensitivity pixels in the sensor.
A second type of HDR cameras extends the dynamic range by merging image data of two or more images that have been captured with different exposure times (e.g. one short exposure time for capturing details in the bright part of the image and one long exposure time for capturing details in the dark part of the image). In this way, a single high-dynamic range image can be created. It is readily understood that the two or more images have to be captured within a short time period to avoid generation of motion artifacts. In this second type of HDR cameras, properly controlling the exposure time used for the capture of the single images that are to be merged into a high-dynamic range image is of crucial importance for the quality of the merged high-dynamic range image.